Lesson 4

Question 1

What is the ‘All or Nothing Law’?

Answer 1

An action potential is either initiated or it is not and it is always the same size

Question 2

How does size of stimulus affect action potential?

Answer 2

• Doesn’t affect size or speed of impulse
• Larger stimulus will create a greater frequency of impulses

Question 3

What is the threshold for action potential to be initiated?

Answer 3

• -55mV

Question 4

What is size of action potential?

Answer 4

• +40mV

Question 5

Describe nerve impulse transmission in a non-myelinated neurone

Answer 5

• Na+ rapidly diffuse into axon during depolarisation and diffuse down conc. grad. to next part of axon
• Adjacent section of membrane depolarised so voltage-gated Na+ channels open in these areas
• This repeats, spreading wave of depolarisation along axon

Question 6

How is refractory period initiated?

Answer 6

Site of initial action potential, Na+ channels are inactivated and cannot reopen until resting potential re-established

Question 7

What is myelin?

Answer 7

Lipid-type substance that is impermeable to ions

Question 8

What do Schwann cells do?

Answer 8

Surround axon, forming myelin sheath that acts as electrical insulator

Question 9

Why do nerve impulses jump from one node of Ranvier to next and what does this do!

Answer 9

• Due to properties of myelin
• So depolarisation and action potential only occur at nodes where there are more ion channels and Na+/K+ pumps
• This causes Saltatory Propagation

Question 10

What is Saltatory Propagation?

Answer 10

• When impulse jumps between nodes
• Local currents caused by Na+ diffusion are elongated and speed of transmission is increased